This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by a carbonylation reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is Group VIIIB metal, i.e., a metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
Further developments in the carbonylation reaction, including the use of such cocatalysts as cobalt pentadentate complexes and terpyridines, are disclosed in U.S. Pat. Nos. 5,231,210, 5,284,964 and 5,399,734. These patents also disclose the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package.
The above-identified patents relate in general to carbonylation methods in which the Group VIIIB metal (most often palladium) component of the catalyst is present at a relatively high level, typically on the order of 500 ppm of palladium based on hydroxyaromatic compound. A further patent, U.S. Pat. No. 5,498,789, discloses the use of a catalyst comprising a mixture of elemental or combined palladium, a lead compound and a quaternary ammonium or phosphonium halide. Palladium levels are disclosed as being roughly equivalent to those noted above or, in some instances, considerably lower, typically on the order of 50 ppm. Yields are expressed conventionally (i.e., in percentage) or in terms of "turnover number", which is the number of moles of diaryl carbonate formed per gram-atom of palladium present.
The commercial viability of the carbonylation reaction would be greatly increased if product yield therein could be improved. The relatively low yields disclosed in the working examples of the above-identified patents include some in the single digits. It is evident, therefore, that methods to improve the yield are needed.